Sigma delta (sometimes referred to as “delta sigma”) modulation is a method for encoding analog signals into digital signals. Sigma delta analog-to-digital converters (ADCs) are used in a variety of devices and sometimes used in the receivers of software defined radios (SDRs) and similar devices.
Receivers in software defined radios must often be able to receive and process signals of multiple bandwidths. In some applications, the receiver and, therefore, the analog-to-digital converter is required to be able to process both narrowband and broadband signals. Generally, narrowband refers to communications that use frequency content within the coherence band of a frequency channel, and broadband refers to the use of frequency ranges beyond those used by narrowband communications. Narrowband and broadband communications may have conflicting hardware requirements. For example, to receive and process narrowband signals, a receiver circuit should have a high dynamic range and low switching noise (for example, kT/C noise) (where k is Boltzmann's constant in joules per Kelvin, T is the capacitor's absolute temperature, and C is the capacitor's capacitance in farads). To achieve low switching noise, a receiver may need to incorporate large capacitors. On the other hand, to receive and process broadband signals, a receiver circuit should run at high speeds, while a relatively modest dynamic range may be sufficient. To run at high speeds, small capacitors are preferred.
Analog-to-digital converters are often implemented using switched capacitor circuits. To handle different bandwidths, the number of capacitors can become quite large making the analog-to-digital converter complex. Having a large number of capacitors can also increase costs. With conventional approaches to receiver design, it becomes difficult to handle multiple bandwidths without significant compromise.
Accordingly, there is a need for a method and apparatus for adjusting the bandwidth of a sigma delta converter.
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